The Conversion of the Digital Codes and Equivalent Voltage

The output of the FS501 ADC is SUM1<23:0>, which is a 24-bit 2’s complement number.

SUM1<23> is the sign bit; 0 represents a positive number, and 1 represents a negative number. The

decimal point lies in between SUM1<22> and SUM1<21>.

If SUM1<23:0>=0010_1000_0000_0000_0000_0000, the equivalent floating point number is:

Equation 7

625

.

0

125

.

0

5

.

0

2

0

......

2

0

2

0

2

1

2

0

2

1

0000

_

0000

_

0000

_

0000

_

1000

_

10

.

00

22

5

4

3

2

1















































SUM1

If SUM1<23:0>=1101_1111_1111_1111_1111_1111, the equivalent floating point number is:

5000002384

.

0

2

1

......

2

0

2

0

2

0

2

1

(

)

0001

_

0000

_

0000

_

0000

_

0000

_

10

.

00

(

1111

_

1111

_

1111

_

1111

_

1111

_

01

.

11

22

4

3

2

1













































SUM1

From Equation 1, if gain

'

G

equals 1 and reference voltage

ref

V

=1.00000V, the value of SUM1,

0010_1000_0000_0000_0000_0000, can be used to calculate the measured voltage as:

V

62500

.

0

625

.

0

1

V

00000

.

1

'











x

ref

x

D

G

V

v

。

If SUM1=1101_1111_1111_1111_1111_1111, the measured voltage can be calculated:

V

50000

.

0

5000002384

.

0

1

V

00000

.

1

'















x

ref

x

D

G

V

v

。

However, due to the manufacture process drift

'

G

is not exactly equal to 1, and there will be

around

%

1



offset. Similarly the reference voltage source and resistors may affect the reference

voltage

ref

V

, and make

ref

V

not to be exact 1.00000V. Therefore, we have to calibrate the ADC.

Other Control Setting

ENAD(ADG<7>) is the enable control signal for the ADC. It is 1 to enable the ADC; it is 0 to turn off

the ADC and can save power.

CPVR(ADG<6>) is the enhancement mode for resistance measuring. It is set to 1 to improve the

linearity when measuring resistance.